For many years sheet-like cementitious products containing high tensile modulus fibrous reinforcements, such as asbestos, have been produced to increase tensile strength and modulus. Such reinforced products were made by mixing the reinforcement with a cement composition and water to form a viscous slurry which was cast or molded to form the desired rigid shape.
The resulting reinforced products, however, were prone to crack upon application of slight bending forces or light impact, such as nailing or shipping and handling.
Recently, sheet-like cementitious products with ductile organic fiber reinfocements provided improvements in the products and in production processes. Improvements in products resulted from investigations of the role of composition and physical form of matrix material and reinforcements on properties. Insufficient investigation was directed, however, to possible damaging effects of production steps on final properties. Also, development of effective continuous processes received little attention.
In the prior art disclosures, Alford, in U.S. Pat. No. 4,528,238, July 9, 1985, discloses a method of making a structure with a fiber-reinforced, moldable, cementitious composition by mixing a viscous cement matrix mixture and a water-insoluble fibrous material, in which water is present in the matrix material at not more than 25 percent of the total weight of the cement. The teaching is directed to mixing a viscous slurry of cement into which the fiber is pressed.
Schupack, in U.S. Pat. No. 4,617,219, Oct. 14, 1986, discloses products with randomly oriented, organic fibrous reinforcements in a cement matrix, the product being produced by pressing the fabric into the slurry, either in molding trays or by a continuous basis on rolls. The patent discloses that the degree to which panels can be bent depends upon a number of factors, principally on thickness. Low-radius bending is achieved with very fine cracks which depend upon proper fabric choice.
Currie, et al., in U.S. Pat. No. 4,578,301, Mar. 25, 1986, disclose a composite structure of a water-hardened cement matrix reinforced with layers of parallel fiber strands, the structure being formed by forcing a cement slurry into the strands.
Tesch, in U.S. Pat. No. 4,495,235, July 17, 1985, discloses a process of laying wet cement between two fabric layers followed by needling the fabric layers together through the cement before the cement sets.
Nicholls, in U.S. Pat. No. 4,446,083, May 1, 1984, discloses a method of construction of a cement roof or enclosure by a sequence involving first spreading a layer of dry cement-aggregate mix on a fabric which is then supported on an inflated membrane and thereafter adding water. This method is adapted to in situ formation of a rigid, curved, concrete roof structure.
The foregoing disclosures as well as similar disclosures in several foreign patent publications are indicative of advances in the art which occurred in recent work on ductile reinforcements. A conclusion to be drawn from this work is that a difference exists between the roles of rigid, non-ductile reinforcements in plastic matrices and that of ductile reinforcements in rigid matrices. The former role relies upon the added strength of the reinforcement. The latter role is reinforcement to cushion and dissipate applied compressive and tensile stresses. Ductile reinforcements prevent major crack formation caused by bending, tension, and compression, including impact.